The constant demand from residential users of a higher transfer rate, along with the reduction of costs per user of access architectures, have made telecommunication operators think about the replacement of the traditional point-to-point access architectures based on the conventional collection telephone pair with fully passive PON (Passive Optical Networks) point-to-multipoint optical fibre architectures.
Likewise, GPON (Gigabit-Capable Passive Optical Network) technology, EPON (Ethernet Passive Optical Network) and their future speed variants (XGPON-1, XGPON-2, EPON NG), allow offering users elevated bandwidths (above 100 Mbps) and containing the network infrastructure deployment costs for being from a shared media.
There are no active devices in a passive optical network between the OLT (Optical Line Terminal) device located in the telephone central office and the user device or ONT (Optical Network Terminator) located at the domicile of the user.
Alternatively, passive optical devices, known as optical dividers or splitters, are introduced which in the downward direction, distribute the optical signal to all of the users within a tree-and-branch architecture using a certain wavelength (normally 1490 nm) and in the upward direction, combine the optical signals from all of the ONTs using a TDM (Time Division Multiplexing) type time division system, usually transmitting in the 1310 nm wavelength.
The use of two supplementary transmission and reception windows allows the use of a monofibre plant for FTTH (Fibre to the Home) deployments instead of the traditional bi-fibre architectures for dedicated transmission and reception.
When an operator decides to offer a PON access system, it usually organizes a two-phase deployment strategy: in the first phase, the installation units of the operators (companies subcontracted by the operators) install the external plant fibre from a dispatcher of the telephone central office until the termination points of the premises of the Common Telecommunication Infrastructures (CTIs) of the subscribers.
During this phase, the installers certify the correct installation of the plant fibres by means of a physical verification using OTDR (Optical Time Domain Reflectometer) tools, modified to test point-multipoint topologies.
The objective of this phase, also known as network ringing, is to guarantee that the optical budget measured by the OTDR is lower than the one available between the OLT and the different ONTs according to the degree of division or splitting selected. Once the fibre testing is completed, the OLTs devices are installed in the telephone central office and the operator waits for the sign-up requests from the different customers.
The second phase of the installation begins when a user requests to be signed-up by the operator, which consists in physically connecting a vacant optical fibre spot of the PON from the terminating cabinet of the CTI premises to the network termination wall socket of the domicile of the subscriber. To achieve the foregoing, a dedicated monofibre laid by the installer company between the two points is used.
The correct installation of this optical fibre interconnection is crucial so the user has no problems with the traffic services to be exchanged with the telephone central office.
One of the effects fought against by installer companies in this phase is the presence of reflections in these last meters of fibre.
A defect in this last fibre leg or in its optical connectors causes a part of the power to reflect and to return to the ONT, thus decreasing the effective power reaching the OLT in the upward link and degrading the provision of the access services.
In order to control this effect, given that the fibre is already connected to the PON infrastructure and usually (due to reasons of simplicity and cost, the fibre network does not have optical filters to limit an injected signal coming from a measuring element) it is not possible to inject a signal coming from a tapped OTDR in the user wall socket because the signal generated in a continuous manner by this last element could disrupt the active elements of the other users, thus interrupting the access service during the sign-up process of new customers.
In the majority of the cases, installers choose to connect an ONT that is similar to the ONT of the user (or the user's own ONT), verifying that the ONT is synchronised with the OLT by deeming the installation as correct, without taking into account whether it is possible to establish the same or their quality of service/experience.
Traditional OTDR-based solutions, such as the one detailed in European patent 99202206.1, exclusively work on a physical level, while the solution proposed in the present specification is capable of working at a physical level and at a services level by measuring the quality thereof as perceived by the user (QoS) and by simulating the same operating conditions to be found by the user.
European patent application 94200753.5 includes an optical circuit to measure the sensitivity of an optical transmission system to reflections. However, in this case, this circuit is not applicable to the detection of reflections in a monofibre PON scenario.
Patent application 200400037 also presents an optical measurement system capable of using reflections to carry out a measurement diagnostic. However, its scope is limited to the use of sensorisation and is not applicable to the field of PON networks.